Pulmonary surfactant is defective in the neonatal Respiratory Distress Syndrome (RDS) and in acute RDS that occurs in patients of all ages. The clinical treatment of RDS involves the use of natural or synthetic pulmonary surfactant. However, the mechanism of formation and structure of an active surfactant film at physiological conditions of near-zero surface tension are still relatively unknown. This research proposal aims to understand the structural aspects of an active surfactant film using epifluorescence microscopy on natural as well as model surfactants. The results obtained will help to understand the mechanism by which the stable surfactant films are formed in vivo and to evaluate the respective role of different surfactant components in the formation and stabilization of these films under physiological conditions.